Manufacturing device of foamed thermoplastic resign sheet

ABSTRACT

A foamed thermoplastic resin sheet manufacturing device is provided with (i) an extruder for melting and kneading a thermoplastic resin and a foaming agent to form a melted/kneaded mixture and extruding the melted/kneaded mixture, (ii) a die which is provided at a front end of said extruder and forms the melted/kneaded mixture into a sheet-like foamy thermoplastic resin material, (iii) a vacuum chamber in which the foamy thermoplastic resin material extruded through said die expands under a reduced pressure, and (iv) a facing-wall section composed of a pair of walls of said vacuum chamber which face each other in a thickness direction of the foamy thermoplastic resin material, at least one of the walls in pair being a movable wall provided so as to move in directions in which said walls approach and separate each other. By this arrangement, foamed thermoplastic resin sheets with various thicknesses can be produced.

FIELD OF THE INVENTION

[0001] The present invention relates to a foamed thermoplastic resinsheet manufacturing device which performs expansion under a reducedpressure.

BACKGROUND OF THE INVENTION

[0002] As a conventional method for producing a foamed thermoplasticresin sheet (hereinafter sometimes referred to simply as “sheet”), amanufacturing method wherein a thermoplastic resin and a foaming agentare melted and kneaded in an extruder and extruded through a die intounder the atmospheric pressure has been well known. To obtain a highlyfoamed sheet by this manufacturing method, however, there areinconveniences that a great amount of a foaming agent is needed, andthat cells become more coarse as the sheet expands, therebydeteriorating a strength of the sheet.

[0003] As a method which has such inconveniences solved, a method inwhich the foamy thermoplastic resin material extruded from an extruderis passed through a vacuum device so that the foamy thermoplastic resinis further expanded has been practiced.

[0004] For example, the Japanese Publication for Laid-Open PatentApplication No. 54215/1990 (Tokukohei 2-54215) (the Japanese Patent No.1639854) discloses an arrangement wherein a roll-like haul-off machineis installed in a vacuum chamber so that the sheet-like foamythermoplastic resin material is extruded through a die to the vacuumchamber so as to expand and the material thus foamed is hauled by thehaul-off machine.

[0005] The Japanese Examined Patent Publication 29328/1983 (The JapanesePatent No. 1199174) discloses an arrangement in which a sealing memberfor ensuring reduction of a pressure in a vacuum chamber is provided atan outlet of the vacuum chamber and a roll-like haul-off machine isinstalled behind the vacuum chamber so as to haul a sheet-like foamythermoplastic resin material which is extruded through a die to thevacuum chamber thereby expanding.

[0006] Incidentally, in the present specification, a thermoplastic resinexpanded under a reduced pressure at a first expansion stage is referredto as “foamy thermoplastic resin,” and the foamed thermoplastic resinfurther expanded under the reduced pressure to a completely expandedstate, obtained after or immediately before curing, is referred to as“foamed thermoplastic resin.” A thermoplastic resin in a state of beingexpanded under a reduced pressure is to be classified as the former“foamy thermoplastic resin.” By the conventional arrangements describedabove, foamed thermoplastic resin sheets with only one thickness areproduced. Therefore, to produce foamed thermoplastic resin sheets withvarious thicknesses, a discrete manufacturing device has to be preparedfor each thickness.

[0007] The manufacturing device disclosed by Tokukohei 2-54215, however,has a drawback in that installment of the haul-off machine in the vacuumchamber causes the sealing mechanism in the vacuum chamber to becomecomplicated, and makes the manufacturing device bulkier.

[0008] Further, regarding the manufacturing method disclosed byTokukosho 58-29328, since the foamed sheet, while being hauled by thehaul-off machine, pushes the sealing member at the outlet of themanufacturing device, cells in the foamed sheet are crushed and surfacesof the sheet are scarred. Note that the drawback in that cells of thefoamed sheet are crushed is more remarkable in the case of Tokukohei2-54215.

[0009] Therefore, though usually cells growing long in the thicknessdirection of the sheet are obtained in the case of expansion under areduced pressure, such an effect of growth of cells due to pressurereduction is not sufficiently achieved in the foregoing prior art, andcells emerging in this case are cells growing long in the directionorthogonal to the sheet thickness direction, that is, in the sheet widthdirection or in the extrusion direction, like in the aforementionedcase. Presence of a number of such cells in a sheet leads to a problemthat the sheet cannot be made thicker.

SUMMARY OF THE INVENTION

[0010] The present invention was made in light of the foregoingproblems, and the object of the present invention is to provide amanufacturing device of a foamed thermoplastic resin sheet which iscapable of manufacturing foamed thermoplastic resin sheets with variousthicknesses.

[0011] To achieve the foregoing object, a manufacturing device of afoamed thermoplastic resin sheet of the present invention ischaracterized by comprising (i) an extruder for melting and kneading athermoplastic resin and a foaming agent to form a melted/kneadedmixture, and extruding the melted/kneaded mixture, (ii) a die providedat a front end of the extruder, for forming the melted/kneaded mixtureinto a sheet-like foamy thermoplastic resin material, (iii) a vacuumchamber in which the foamy thermoplastic resin material extruded throughthe die expands under a reduced pressure, and (iv) a facing-wall sectioncomposed of a pair of walls of the vacuum chamber which face each otherin a thickness direction of the foamy thermoplastic resin material, atleast one of the walls being a movable wall provided so as to move indirections in which the walls approach and separate each other.

[0012] With the foregoing arrangement, the melted/kneaded mixture of thethermoplastic resin and the foaming agent is extruded through the die byan extruding operation by the extruder, thereby becoming a sheet-likefoamy thermoplastic resin material. The foamy thermoplastic resinmaterial further expands under a reduced pressure in the vacuum chamber,thereby becoming the formed thermoplastic resin sheet.

[0013] Here, the thickness of the foamed thermoplastic resin sheet isdetermined depending on a dimension of the vacuum chamber in the foamedthermoplastic resin sheet thickness direction. In other words, thefoamed thermoplastic resin sheet expands to a thickness equivalent tothe dimension of the vacuum chamber in the foamed thermoplastic resinsheet thickness direction.

[0014] On the other hand, in the facing-wall section composed of a pairof walls of the vacuum chamber which face each other in the thicknessdirection of the foamy thermoplastic resin material, at least one of thewalls is a movable wall provided so as to move in directions in whichthe walls approach and separate each other. Therefore, the thickness ofthe foamed thermoplastic resin sheet in the vacuum chamber can bechanged by moving the movable wall, and as a result, the foamedthermoplastic resin sheet can be formed to various thicknesses.

[0015] Another object of the present invention is to provide amanufacturing device of a foamed thermoplastic resin sheet which iscapable of producing a thick sheet whose surface condition is excellentand whose good foaming state under a reduced pressure is maintainedthereby ensuring a high foaming ratio.

[0016] To achieve the foregoing object, a manufacturing device of afoamed thermoplastic resin sheet of the present invention, arranged asabove, is further arranged as follows.

[0017] Namely, a sealing member is provided at least on the movable wallin the facing-wall section, at an outlet of the vacuum chamber, so as toseal a space between the wall and a foamed thermoplastic resin sheetwhich is obtained as a result of expansion of the foamy thermoplasticresin material under a reduced pressure and comes out of the vacuumchamber.

[0018] The foregoing arrangement ensures that a set desired pressure iskept in the vacuum chamber, without a great force applied from thesealing members to the foamy thermoplastic resin material and the foamedthermoplastic resin sheet, at all times since the start of pressurereduction.

[0019] For example, the gap between the walls in pair of the facing-wallsection is narrowed in a period prior to pressure reduction in thevacuum chamber, i.e., when expansion of the sheet-like foamythermoplastic resin material due to pressure reduction does not yetstart and the material has a small thickness, whereas the gap is widenedafter the pressure reduction in the vacuum chamber starts therebycausing the sheet-like foamy thermoplastic resin material to expand dueto the reduced pressure and have a greater thickness, that is, when thesheet-like foamy thermoplastic resin material becomes the foamedthermoplastic resin sheet. Therefore, before and after the start of thepressure reduction in the vacuum chamber, the respective relationshipsbetween the walls and the sheet-like foamy thermoplastic resin materialor the foamed thermoplastic resin sheet, that is, the respectiveposition relationship between the sealing members and the sheet-likefoamy thermoplastic resin material or the foamed thermoplastic resinsheet in the thickness direction of the sheet-like material (or sheet)can be maintained substantially unchanged. This allows the sealingmembers to be made of a soft and flexible material, and the sealingmembers thus formed by no means scar the surfaces of the foamedthermoplastic resin sheet, nor crush cells in the foamed thermoplasticresin sheet.

[0020] In contrast, by an arrangement in which the facing- wall sectiondoes not have a movable wall, it is required to make the sealing membersof a material with a relatively high rigidity so as to maintain adesirably reduced pressure in the vacuum chamber before and after achange of the thickness of the sheet due to expansion under the reducedpressure. In this case, an excessively great force is applied from thesealing members to the foamed thermoplastic resin sheet with a greaterthickness due to expansion under the reduced pressure, whereby thesurfaces of the foamed thermoplastic resin sheet tend to be scarred bythe sealing members, or cells in the sheet tend to be crushed.

[0021] Consequently, with a simple arrangement of the present inventionin which sealing members are provided as sealing means for maintainingthe reduced pressure in the vacuum chamber, a sheet whose surface is notscarred by the sealing members and which maintains a good foaming stateunder a reduced pressure thereby having a high foaming ratio and beingformed thick can be easily obtained.

[0022] For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1(a) is a schematic cross-sectional view illustrating anarrangement of a foamed thermoplastic resin sheet manufacturing devicein accordance with an embodiment of the present invention.

[0024]FIG. 1(b) is a plan view of the same.

[0025]FIG. 2 is a schematic cross-sectional view taken along an A-Aarrow line in FIG. 1(b).

[0026]FIG. 3(a) is a schematic vertical cross-sectional viewillustrating a shape of POROUS ELECTROCAST which is adaptable as aporous material used in an expanding zone and a cooling zone shown inFIG. 1 (a), as well as illustrating how a surface of the same is madeuneven.

[0027]FIG. 3(b) is a schematic vertical cross-sectional viewillustrating another example of the arrangement shown in FIG. 3(a).

[0028]FIG. 4(a) is a schematic vertical cross-sectional viewillustrating another example of a manner how to make the surface unevenas shown in FIG. 3(a).

[0029]FIG. 4(b) is a schematic vertical cross-sectional viewillustrating still another example of a manner how to make the surfaceuneven as shown in FIG. 3(a).

[0030]FIG. 5(a) is a schematic vertical cross-sectional viewillustrating a state of the manufacturing device shown in FIG. 1(a) uponinitialization of extrusion of a sheet-like foamy thermoplastic resinmaterial.

[0031]FIG. 5(b) is a schematic vertical cross-sectional viewillustrating a state subsequent to the state shown in FIG. 5(a), priorto pressure reduction of the vacuum chamber.

[0032]FIG. 6 is a schematic vertical cross-sectional view illustratinganother example of the manufacturing device shown in FIG. 1(a), in whicha die lip projects into the vacuum chamber.

[0033]FIG. 7 is a schematic cross-sectional view illustrating anarrangement of a foamed thermoplastic resin sheet manufacturing devicein accordance with another embodiment of the present invention.

[0034]FIG. 8(a) is a schematic vertical cross-sectional viewillustrating a state of the manufacturing device shown in FIG. 7 uponinitialization of extrusion of a sheet-like foamy thermoplastic resinmaterial.

[0035]FIG. 8(b) is a schematic vertical cross-sectional viewillustrating a state which is subsequent to the state shown in FIG. 8(a)and prior to pressure reduction of the vacuum chamber.

[0036]FIG. 9 is a schematic vertical cross-sectional view illustratingan arrangement of a foamed thermoplastic resin sheet manufacturingdevice in accordance with still another embodiment of the presentinvention.

[0037]FIG. 10(a) is a schematic vertical cross-sectional viewillustrating an arrangement of a foamed thermoplastic resin sheetmanufacturing device in accordance with still another embodiment of thepresent invention.

[0038]FIG. 10(b) is a plan view of the same.

[0039]FIG. 11(a) is a schematic vertical cross-sectional viewillustrating an arrangement of a foamed thermoplastic resin sheetmanufacturing device in accordance with still another embodiment of thepresent invention.

[0040]FIG. 11(b) is a plan view of the same.

[0041]FIG. 12 is an explanatory view illustrating a method for measuringcell diameters of a foamed thermoplastic resin sheet produced by themanufacturing device shown in FIG. 1(a) in accordance with an example ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] [First Embodiment]

[0043] The following description will explain an embodiment of thepresent invention while referring to FIGS. 1 through 6.

[0044] A manufacturing device of the foregoing foamed thermoplasticresin sheet 6 is equipped with an extruder 1, a head part 2, a die 3, avacuum chamber 4, and a haul-off machine 5, as shown in FIG. 1(a). Thevacuum chamber 4 is installed inside a molding section 9.

[0045] The extruder 1 melts and kneads a foaming agent and athermoplastic resin at a temperature previously set suitable to thethermoplastic resin and the foaming agent used, and extrudes the mixtureobtained toward the head part 2. It is preferable that the extruder 1has a single shaft structure, in the case where the foaming agent andthe thermoplastic resin are kneaded at a low temperature.

[0046] The temperature of the extruder 1 is set, for example, in thecase where the thermoplastic resin used is polypropylene resin, so thata temperature of a melt of polypropylene resin (kneaded material) as afoamy thermoplastic resin material is not higher than 180° C. in thevicinity of an outlet of the extruder 1. This is because outgassingoccurs when the temperature of the melt of the polypropylene resinexceeds 180° C.

[0047] The head part 2 is positioned at the outlet of the extruder 1,and a screen mesh usually used for molding by extrusion is used. In thecase where, however, the thermoplastic resin used is a resincharacterized by remarkable temperature rise when being shorn, a screenmesh is not used.

[0048] The die 3 is a sheet die for processing the foamy thermoplasticresin material extruded from the extruder 1 through the head part 2 intoa sheet form. Generally, the sheet die is arranged so that temperatureand pressure are adjustable. The die 3 has a die lip 3 a as an outletthrough which the resin is discharged.

[0049] The haul-off machine 5 positioned on a side to an outlet of thevacuum chamber 4 so as to haul the foamed thermoplastic resin sheet 6through the outlet of the vacuum chamber 4. The haul-off machine 5 iscomposed of not less than one pair of rolls 5 a facing each other so asto be capable of nipping the foamed thermoplastic resin sheet 6therebetween. The rolls 5 a are movably provided in directions such thata gap therebetween is narrowed and widened. For example, the rolls 5 aare movably provided so as to move as a movable upper wall 12 and amovable lower wall 13 which will be described later move. Incidentally,each roll 5 a is preferably arranged so that a temperature thereof isadjustable by cooled water. In the place of the roll-type haul-offmachine 5, another type haul-off machine (for example, a belt-type one)conventionally used in manufacture of resin sheets may be used.

[0050] A hauling speed of the haul-off machine 5 should be appropriatelyset according to a foaming ratio, a thickness, a resin composition, andthe like of the foamed thermoplastic resin sheet 6, but usually it isset to 1 to 3 m/min.

[0051] The vacuum chamber 4 is used for further expanding the sheet-likefoamy thermoplastic resin material (hereinafter referred to assheet-like foamy thermoplastic resin material 6 a, so as to bedistinguished from the foamed thermoplastic resin sheet 6 in thecompletely expanded stage) extruded from the die 3 under a reducedpressure. The vacuum chamber 4 has such a structure as is capable ofcooling the sheet-like foamy thermoplastic resin material 6 a afterfurther expanding the sheet-like foamy thermoplastic resin material 6 a.

[0052] An inlet side part of the vacuum chamber 4 is an expanding zone 7in which the sheet-like foamy thermoplastic resin material 6 a extrudedfrom the die 3 is expanded under a reduced pressure, and an outlet sidepart thereof is a cooling zone 8 in which the sheet-like foamythermoplastic resin material 6 a expanded in the expanding zone 7 iscooled so as to be cured. Incidentally, there is no need to strictlyseparate the expanding zone 7 and the cooling zone 8 with use of aseparating board or the like. The expanding zone 7 has a pressure nothigher than that of the cooling zone 8, and also has a function ofcooling the sheet-like foamy thermoplastic resin material 6 a afterexpanding it. The cooling zone 8 is a zone in which the sheet-like foamythermoplastic resin material Ga is cured.

[0053] Evacuation of the vacuum chamber 4 is carried out by a vacuumpump 17 connected to the expanding zone 7, through inner walls 14 havingvacuum ports, which are provided in the expanding zone 7 and the coolingzone 8. The inner walls 14 will be described later. Adjustment of thereduced pressure is conducted by a pressure regulator 15 and a vacuumbreaker 16. In adjustment of the reduced pressure, the pressureregulator 15 and the vacuum breaker 16 may be used in combination, oralternatively, one of them may be used. Here, the expanding zone 7 andthe cooling zone 8 are evacuated by one evacuating pump 17 at the sametime, but alternatively, another evacuating pump 17 may be provided tothe cooling zone 8 so that pressures of the zones 7 and 8 areindependently adjusted. In addition, it is preferable that eachevacuating line is equipped with a pressure adjusting valve so that thereduced pressure can be adjusted. Applicable as the pressure adjustingvalve is a generally-used type, such as a type which controls an inletpressure of the evacuating pump 17 by changing an aperture thereof withuse of a pressure detector and a pressure transmitter, or that of a typewhich per se controls the pressure by monitoring a pressure gauge.

[0054] Regarding pressure in the vacuum chamber 4, a differentialpressure of about 200 mmHg (a difference from the atmospheric pressure)is required in the case where the thermoplastic resin used is forexample polypropylene resin. The differential pressure is preferably setnot less than 300 mmHg, or more preferably 350 to 700 mmHg. An optimalpressure, however, varies depending on the thermoplastic resin and thefoaming agent used, and also varies with a desired foaming ratio of thefoamed thermoplastic resin sheet 6.

[0055] The following description will explain the molding section 9 andthe vacuum chamber 4 in more detail.

[0056] As shown in FIG. 1(a) and FIG. 2 which is a view (the sheet-likefoamy thermoplastic resin material 6 a is omitted) of a cross sectiontaken along an A-A arrow line in FIG. 1(b), the molding section 9includes the movable upper wall 12 and the movable lower wall 13 whichare vertically movable, in a space enclosed by outer walls 11 in abox-like form. The space enclosed by the movable upper and lower walls12 and 13 and the outer walls 11 constitutes the vacuum chamber 4. Inthe present embodiment, a wall surface 12 a which is a lower surface ofthe movable upper wall 12 and a wall surface 13 a which is an uppersurface of the movable lower wall 13 are plane. The movable upper wall12 and the movable lower wall 13 have the inner walls 14 on their sidesto the vacuum chamber 4, and surfaces of the inner walls 14 constitutethe aforementioned wall surfaces 12 a and 13 a, respectively.

[0057] The molding section 9 is equipped with a movable wall drivingdevice 23 for moving the movable upper wall 12 and the movable lowerwall 13. The movable wall driving device 23 has a plurality of screws 24which vertically pierce the outer wall 11, and ends of the screws 24 arefixed to an upper surface of the movable upper wall 12. Portions of thescrews 24 projecting from the outer wall 11 are fit to internal threads(not shown) formed in sprockets 25. The sprockets 25 are rotatablyprovided on an outer surface of the outer wall 11. On the outer surfaceof the outer wall 11, there is provided a handle 26 which is rotatable,and a sprocket 27 is made rotatable by the handle 26. To the sprockets25 and 27, a belt 28 with teeth is applied, as shown in FIG. 1(b). Thebelt 28 may be substituted by a chain.

[0058] On the lower side of the movable lower wall 13, likewise, thereare provided screws 24, sprockets 25, a sprocket 27, and a belt 28. Onthe lower side, however, no handle 26 is provided, and rotation of thehandle 26 on the upper side is transmitted to the sprocket 27 on thelower side by a drive transmitting mechanism not shown. Alternatively,however, an independent movable wall driving device 23 identical to thaton the upper side may be provided on the lower side as well, so that themovable upper wall 12 and the movable lower wall 13 are independentlymoved by the handles 26, respectively, i.e., by the movable drivingdevices 23, respectively.

[0059] With the aforementioned arrangement, the movable upper wall 12and the movable lower wall 13 are simultaneously moved upward/downwardby rotating the handle 26. In this case, the movable upper wall 12 andthe movable lower wall 13 are moved in opposite directions. Thus, a gapbetween the wall surface 12 a of the movable upper wall 12 and the wallsurface 13 a of the movable lower wall 13, that is, a height of thevacuum chamber 4 equivalent to a thickness of the foamed thermoplasticresin sheet 6 is adjustable.

[0060] Incidentally, movement of the movable upper wall 12 and themovable lower wall 13 is preferably performed in a state in which themovable upper and lower walls 12 and 13 slant in neither the extrusiondirection nor the width direction of the foamed thermoplastic resinsheet 6, being parallel with each other at all times.

[0061] Furthermore, in the present embodiment, the wall surface 12 a ofthe movable upper wall 12 and the wall surface 13 a of the movable lowerwall 13 move equal distances, respectively, with respect to a centralposition of the foamed thermoplastic resin sheet 6 in the thicknessdirection at the die lip 3 a.

[0062] Furthermore, an arrangement of the upper wall driving device 23is not limited to the foregoing screw type. Known arrangements, such asan arrangement in which hydraulic cylinders are used, may be adapted.The foregoing screw type is suitable to a small-scale manufacturingdevice, while the hydraulic cylinder type is suitable to a large-scalemanufacturing device.

[0063] Furthermore, in the present embodiment, the movable upper andlower walls 12 and 13 are provided, but alternatively, only one of themmay be movably provided.

[0064] On edges of the movable upper and lower walls 12 and 13 on thevacuum chamber 4 outlet side, blade-like sealing members 31 areprovided, respectively. The sealing members 31 are provided on themovable upper and lower walls 12 and 13, respectively, for example, froma right or left end to the other end on each wall edge. The sealingmembers 31 seal the vacuum chamber 4 so that reduction of pressure inthe vacuum chamber 4 is ensured. In a state in which the sheet-likefoamy thermoplastic resin material 6 a or the foamed thermoplastic resinsheet 6 are present between the sealing members 31, the sealing members31 curve or bend in a transport direction of the same. The sealingmembers 31 are soft and flexible, made of, for example, rubber.

[0065] Another sealing member 31 may also be provided in the thicknessdirection of the foamed thermoplastic resin sheet 6 on both sides to theedges of the sheet 6, in addition to the foregoing positions. Thesealing members 31 in the thickness direction are fixed to the outerwalls 11. Further, the sealing members 31 may be provided either on anupper side or on a lower side to the sheet-like foamy thermoplasticresin material 6 a or the foamed thermoplastic resin sheet 6, in a widthdirection.

[0066] In the vacuum chamber, the expanding zone 7 is a zone in whichthe sheet-like foamy thermoplastic resin material 6 a extruded by thedie 3 is to be further expanded with the temperature thereof adjustedsuitable for foaming. The pressure in the expanding zone 7 is reduced.The expanding zone 7 is instantaneously widened to a width W₂corresponding to the gap between the wall surfaces 12 a and 13 a, from athickness corresponding to the aperture thickness of the die lip 3 a,during manufacture of the foamed thermoplastic resin sheet 6.

[0067] Evacuation of the expanding zone 7 is carried out through amember having vacuum ports, which is provided on a part or a whole ofwalls surrounding the expanding zone 7.

[0068] The evacuation may be carried out by evacuating air in thethickness direction, or in the direction orthogonal to the thicknessdirection (in the width direction), or in both the directions. Toevacuate in the thickness direction is preferable.

[0069] It is further preferable that the movable upper wall 12 havevacuum ports with a diameter of 20 mm or smaller each on end portionsthereof in the width direction so that the pressure is reduced in thethickness direction. In the case where the diameter of the vacuum portsis more than 20 mm, melted resin tends to clog up, thereby sometimescausing the hauling operation of the foamed thermoplastic resin sheet 6by the haul-off machine 5 to stop.

[0070] In the present embodiment, the movable upper and lower walls 12and 13 are made of a material having the vacuum ports. As such amaterial, a porous material such as a sintered alloy or a porouselectrocast shell is suitably adapted.

[0071] An example of the porous electrocast shell as one of the porousmaterials is PORASU DENCHU (POROUS ELECTROCAST, Japanese registeredtrademark), and FIGS. 3(a) and 3(b) schematically illustrate crosssections of the inner wall 14 made of POROUS ELECTROCAST. In POROUSELECTROCAST, a vent hole H broadens toward a reverse side. Therefore, ithas a property of being hardly clogged up, and has a low gas evacuationresistance. POROUS ELECTROCAST is an electrocast type which causes metalreversing by plating a model with a metal such as nickel.

[0072] The inner wall 14 shown in FIG. 3(b) has a thicker surfaceportion as compared with that of the inner wall 14 shown in FIG. 3(a).Therefore, the surface thereof is more easily processed, and a pressureresistance thereof increases. The number of ports of POROUS ELECTROCASTis usually 3/cm² to 7/cm², and preferably, 3/cm² to 5/cm². If the numberis greater than the above, a strength gradually decreases.

[0073] It is required to form each vacuum port of the inner wall 14 notgreater than 100 μm, preferably not greater than 50 μm, or morepreferably not greater than 30 μm. In the case where the vacuum portsare great, additives, melted resin, and decomposited resin of thethermoplastic resin used tend to clog up the vacuum ports of the innerwalls 14, and a large-scale vacuum pump 17 becomes needed to keep adesired reduced pressure.

[0074] Further, each inner wall 14 positioned in the expanding zone 7 iskept to a desired temperature by cooled water running through a cooledwater path (temperature adjusting means, cooling medium path) 18 whichis buried in the inner wall 14. In the case where the inner walls 14 aremade of a metal with a high thermal conductivity, a great cooling effectcan be achieved. The cooled water path 18 may be one line to cool thewhole expanding zone 7. To keep good the foaming state of the sheet-likefoamy thermoplastic resin material 6 a, however, it is preferable that aplurality of independent lines are juxtaposed in the extrusion directionof the sheet-like foamy thermoplastic resin material 6 a.

[0075] Incidentally, means for adjusting the temperature of theexpanding zone 7 is not particularly limited, provided that it iscapable of adjusting the temperature. For example, an arrangement inwhich air is blown into the expanding zone 7 is acceptable. In the casewhere this arrangement is adapted, it is possible to maintain thereduced pressure in the vacuum chamber 4 as a whole by carrying outvacuum drawing which overwhelms the air blowing.

[0076] Here, since the whole, or substantially whole, surfaces of thesheet-like foamy thermoplastic resin material 6 a is brought intocontact with the inner walls 14 in the expanding zone 7, the temperatureof the inner walls 14 is fully conducted to the sheet-like foamythermoplastic resin material 6 a. As a result, a temperature adjustmenteffect is improved.

[0077] In the foregoing arrangement of the expanding zone 7, thesheet-like foamy thermoplastic resin material 6 a is dragged while beingin contact with the inner walls 14 in the expanding zone 7. Therefore,if a contact area thereof is large, the sheet 6 a tends to be scarred onits surfaces, as well as smooth hauling of the sheet 6 a cannot beachieved.

[0078] To solve these problems, a number of small protuberances 21 areformed on surfaces of the inner walls 14 in the expanding zone 7, thatis, on the wall surfaces 12 a and 13 a in the expanding zone 7, as shownin FIGS. 3(a) and 3(b). The protuberances 21 shown in FIGS. 3(a) and3(b) have curving surfaces and are independently formed. By making thewall surfaces 12 a and 13 a uneven with the protuberances 21, an area ofthe wall surfaces 12 a and 13 a in contact with the sheet-like foamythermoplastic resin material 6 a (for example, the sheet-like foamypolypropylene resin material) or the foamed thermoplastic resin sheet 6is reduced, whereby smooth sliding of the sheet-like foamy thermoplasticresin material 6 a or the foamed thermoplastic resin sheet 6 isachieved.

[0079] The area of the wall surfaces 12 a and 13 a in contact with thesheet-like foamy thermoplastic resin material 6 a is preferably reducedto not less than 10 percent and not more than 80 percent the area in thecase where the protuberances do not exist. In the case where the area ofthe wall surfaces 12 a and 13 a in contact with the sheet-like foamythermoplastic resin material Ga is less than 10 percent, it is too smallto ensure that the sheet-like foamy thermoplastic resin material 6 a issufficiently cooled. In the case where the contact area is greater than80 percent, the hauling of the sheet-like foamy thermoplastic resinmaterial 6 a tends to become difficult.

[0080] Furthermore, it is more preferable that the protuberances 21 areplated with, for example, polytetrafluoroethylene (Teflon). This makesthe sliding of the sheet-like foamy thermoplastic resin material 6 asmoother, thereby decreasing probability that the surfaces thereof arescarred.

[0081] Patterns of the protuberances 21 shown in FIGS. 3(a) and 3(b) aremerely examples, and there is no particular limitation on the pattern ofthe protuberances. The crepe pattern, or the crape pattern, for example,is suitably applied. The formation of protuberances in such a patternand the plating of the same are more preferably carried out with respectto, not only the wall surfaces 12 a and 13 a of the expanding zone 7,but also wall surfaces of the outer walls 11 (see FIG. 2) enclosing theexpanding zone 7. Alternatively, however, they may be carried out withrespect to only a part of them. In the case where a porous material isused for drawing a vacuum, it is preferable that the formation ofprotuberances in a pattern and the plating of the same are carried outwith respect to both the metal surfaces and surfaces of the porousmaterial constituting inner wall surfaces of the expanding zone 7, butthe formation of protuberances may be carried out with respect to onlyeither the metal surfaces or the porous material.

[0082] FIGS. 3(a) and 3(b) show arrangements in which the protuberances21 differing in size are provided, but the protuberances 21 formed to auniform size may be provided, as shown in FIGS. 4(a) and 4(b). Regardinga shape of each protuberance 21, the protuberance 21 preferably has acurved surface (see FIGS. 3(a) and 4(a)). The shape is, however, notlimited to this, and it may be a pyramid, or a cone (see FIG. 4(b)).

[0083] An apparent friction coefficient k between the sheet-like foamythermoplastic resin material 6 a and the wall surfaces 12 a and 13 a ispreferably controlled so as to become not more than 0.4, by forming theprotuberances in a desired pattern and plating them with, for example,polytetrafluoroethylene (Teflon). Here, the apparent frictioncoefficient k is defined by the following expression:

(DIFFERENTIAL PRESSURE)×(AREA OF WALL SURFACE 12a(OR 13a))×k=HAULINGFORCE

[0084] where the differential pressure is a differential pressurebetween pressure in the vacuum chamber 4 and the atmospheric pressure.

[0085] The apparent friction coefficient k is preferably not greaterthan 0.35, or more preferably not greater than 0.32. This is becausethat in the case where the apparent friction k is greater than 0.4, thehauling of the foamed thermoplastic resin sheet 6 by the haul-offmachine 5 tends to stop.

[0086] On the other hand, the cooling zone 8 is a zone in which thefoamed thermoplastic resin sheet 6 expanded in the thickness directionin the expanding zone 7 is cooled so as to be cured. The structure fordrawing a vacuum in the cooling zone 8 is identical to that in theexpanding zone 7, and air inside is evacuated through a member withvacuum ports. As the material with the vacuum ports, a porous materialis desirable here as well, and conditions regarding an aperture of thevacuum ports, arrangement of the same, and suitable materials forforming the member, and the like, are the same as those describedrelating to the expanding zone 7. The number of the vacuum ports,however, may be smaller concerning the cooling zone 8 than thatconcerning the expanding zone 7. In the case where POROUS ELECTROCAST isused as a porous material, the number of pores of POROUS ELECTROCAST isnot particularly limited, but it is preferably not more than 3/cm².

[0087] The cooling zone 8 preferably has a lower pressure than thepressure in the expanding zone 7 (has a pressure close to theatmospheric pressure). In this case, an effect of facilitating thehauling of the foamed thermoplastic resin sheet 6 by the haul-offmachine 5 can be achieved.

[0088] In the cooling zone 8 as well, the surfaces of the foamedthermoplastic resin sheet 6 are completely or partially brought intocontact with the inner walls 14 in the cooling zone 8, thereby allowingthe temperature of the inner walls 14 to be fully transmitted to thefoamed thermoplastic resin sheet 6. As a result, a high cooling effectcan be achieved.

[0089] Further, in the cooling zone 8 as well, like in the expandingzone 7, the foamed thermoplastic resin sheet 6 is dragged. Therefore,the surfaces of the inner wall surfaces of the cooling zone 8 are madeuneven, with protuberances 21 being formed on the inner wall surfaces.More preferably, the protuberances are plated.

[0090] Incidentally, means for adjusting the temperature of the coolingzone 8 is not particularly limited, provided that it is capable ofadjusting the temperature. An arrangement in which temperatureadjustment is conducted with use of a cooling medium with a set desiredtemperature, an arrangement in which air is blown into the cooling zone8, or the like is acceptable. In the case where the arrangement in whichair is blown thereto is adapted, it is possible to maintain the reducedpressure in the vacuum chamber 4 as a whole, by carrying out vacuumdrawing which overwhelms the air blowing. In the case where theforegoing arrangement is adapted, particularly, friction of the foamedthermoplastic resin sheet 6 with the inner wall surfaces is reduced,whereby the hauling of the foamed thermoplastic resin sheet 6 is furtherfacilitated.

[0091] At the outlet of the cooling zone 8, it is preferable that thefoamed thermoplastic resin sheet 6 has a central temperature of nothigher than 50° C. which is a central temperature of the sheet-likefoamy thermoplastic resin material 6 a at the outlet of the die 3. By sodoing, it is possible to maintain the cells grown in the thicknessdirection in the formed thermoplastic resin sheet 6 under a reducedpressure.

[0092] In the case where the foamed thermoplastic resin sheetmanufacturing device arranged as described above is used, the foamedthermoplastic resin sheet 6 can be obtained through the followingprocedure.

[0093] First, the foaming agent and the thermoplastic resin are meltedand kneaded by the extruder 1, and thereafter, the kneaded mixture isextruded by the die 3 into a sheet form, thereby being formed into thesheet-like foamy thermoplastic resin material 6 a. Here, themanufacturing device is in a state in which, as shown in FIG. 5(a), themovable upper and lower walls 12 and 13 are positioned by the movablewall driving device (driving means) 23 at upper and lower positions,respectively, so that a gap between the wall surfaces 12 a and 13 a iswider than the thickness of the sheet-like foamy thermoplastic resinmaterial 6 a.

[0094] The sheet-like foamy thermoplastic resin material 6 a extrudedfrom the die 3 reaches the haul-off machine 5 through the vacuum chamber4, becoming ready to be hauled by the haul-off machine 5. Here, therolls 5 a facing each other are in a state in which a gap therebetweenis narrowed to the thickness of the sheet-like foamy thermoplastic resinmaterial 6 a.

[0095] Subsequently, the sheet-like foamy thermoplastic resin material 6a is continuously extruded while the movable upper and lower walls 12and 13 are moved so that the gap between the wall surfaces 12 a and 13 ais reduced to W₁ by the movable wall driving device 23, as shown in FIG.5(b). The gap W₁ is equal to a thickness of the sheet-like foamythermoplastic resin material 6 a extruded from the die 3 when the vacuumchamber 4 is not subject to pressure reduction. In this state, ends ofthe sealing members 31 sufficiently reach the surfaces of the sheet-likefoamy thermoplastic resin material 6 a and covers the outlet of thevacuum chamber 4, thereby ensuring the state in which the vacuum chamber4 is ready to be subject to pressure reduction. Here, the sealingmembers 31 are curved or bent in a direction of transport of thesheet-like foamy thermoplastic resin material 6 a, in contact with thesurfaces of the sheet-like foamy thermoplastic resin material 6 a.

[0096] Thereafter, the pressure in the vacuum chamber 4 is reduced so asto become lower than the atmospheric pressure by not less than 100 mmHg,while the movable upper and lower walls 12 and 13 are moved until a gapbetween the wall surfaces 12 a and 13 a becomes equal to the width W₂,as shown in FIG. 1(a). The gap W₂ is set equal to the desired thicknessof the foamed thermoplastic resin sheet 6 to be produced, and it can bearbitrarily changed. Incidentally, the pressure is preferably reduced bynot more than 700 mmHg. Under the foregoing set conditions, the foamedthermoplastic resin sheet 6 can be smoothly hauled from the vacuumchamber 4.

[0097] With the foregoing pressure reducing operation, the sheet-likefoamy thermoplastic resin material 6 a further expands while goingthrough the expanding zone 7, thereby becoming the foamed thermoplasticresin sheet 6. The foamed thermoplastic resin sheet 6 is cooled to becured when the adjoining cooling zone 8, and subsequently hauled by thehaul-off machine 5. Incidentally, in the case where the foamedthermoplastic resin sheet 6 is continuously produced, the vacuum chamber4 is subject to pressure reduction, while the gap between the wallsurfaces 12 a and 13 a is fixed to W₂.

[0098] As described above, since the present manufacturing device isarranged so that the gap W₂ between the wall surfaces 12 a and 13 a whenthe foamed thermoplastic resin sheet 6 is continuously manufactured isarbitrarily set by the movable wall driving device 23, it is possible toproduce the foamed thermoplastic resin sheet 6 with a desired thickness.Therefore, the manufacturing device can be used for forming the foamedthermoplastic resin sheet 6 to various thicknesses. Thus, themanufacturing device has high flexibility.

[0099] Furthermore, by the movable wall driving device 23, the gapbetween the wall surface 12 a of the movable upper wall 12 and the wallsurface 13 a of the movable lower wall 13 is narrowed in a period priorto pressure reduction in the vacuum chamber 4, i.e., when expansion ofthe sheet-like foamy thermoplastic resin material 6 a due to pressurereduction does not yet start and the material 6 a has a small thickness,whereas the gap is widened after the pressure reduction in the vacuumchamber 4 starts thereby causing the sheet-like foamy thermoplasticresin material 6 a to expand due to the reduced pressure and have agreater thickness, that is, when the sheet-like foamy thermoplasticresin material 6 a becomes the foamed thermoplastic resin sheet 6.Therefore, before and after the start of the pressure reduction in thevacuum chamber 4, the respective relationships between the wall surfaces12 a and 13 a and the sheet-like foamy thermoplastic resin material 6 aor the foamed thermoplastic resin sheet 6, that is, the respectiveposition relationship between the sealing members 31 and the sheet-likefoamy thermoplastic resin material 6 a or the foamed thermoplastic resinsheet 6 in the thickness direction of the sheet-like material 6 a (orsheet 6) can be maintained substantially unchanged. This allows thesealing members 31 to be made of a material with high flexibility, aswell as to be formed to a simple structure. For example, it may beformed to a blade-like shape by using a material such as rubber.

[0100] In other words, in the present embodiment, there is no need tomake the sealing members 31 of a material with a high rigidity or toprovide sealing means of a complicated structure. Doing so is usuallyrequired for reducing that the pressure in the vacuum chamber 4 in thecase where wide gaps exist between the sheet-like foamy thermoplasticresin material 6 a and the wall surfaces 12 a and 13 a respectivelybefore the pressure reduction in the vacuum chamber 4, as is the casewith an arrangement in which the wall surfaces 12 a and 13 a areunmovable.

[0101] With the foregoing arrangement of the present embodiment, such asealing state of the vacuum chamber 4 that the pressure reduction isenabled is maintained by the sealing members 31, while the sealingmembers 31 are prevented from, with a pressing force thereof, scarringthe surfaces of the foamed thermoplastic resin sheet 6, or crushingcells in the foamed thermoplastic resin sheet 6. As a result, the foamedthermoplastic resin sheet 6 is formed without difficulty to have a goodsurface condition and to fully enjoy an effect of cell growth due topressure reduction thereby having the cells therein efficiently grown upin the thickness direction.

[0102] Incidentally, the manufacturing device of the present embodimentmay be arranged so that the die lip 3 a of the die 3 projects to aninlet part of the vacuum chamber 4, as shown in FIG. 6. This arrangementensures prevention of such a problem that, in a state as shown in FIG.5(b) in which the gap between the wall surfaces 12 a and 13 a isnarrowed, a part of the sheet-like foamy thermoplastic resin material 6a gets into a crack between the movable upper or lower wall 12 or 13 andthe die 3, thereby hindering transport of the sheet-like foamythermoplastic resin material 6 a.

[0103] Incidentally, a thermoplastic resin which can be used as amaterial of the foamed thermoplastic resin sheet 6 is not particularlylimited, and any resin which is usually used as extrusion moldingcompound or injection molding compound is adaptable. Examples of suchresins include polyolefin resin such as polyethylene and polypropylene,polystyrene, polyvinyl chloride, polyamide, acrylic resin, polyester,polycarbonate, and a copolymer of any ones of these resins. It ispreferable that polypropylene resin is used as the thermoplastic resin.The polypropylene resin may be any one among a homopolymer, a blockpolymer, and a random polymer. Further, it may be a mixture of the samewith another olefin resin. In this case, the polyolefin to be mixed ispreferably a polyolefin with the number of carbon atoms of not more than10, such as polyethylene or polybutene, and polyethylene resin is mostpreferable among them. In the case where the polypropylene resin andanother polyolefin resin are mixed, a content of the polypropylene isset to 50 wt %.

[0104] An example of more preferable polypropylene resin is a propylenepolymer whose melt strength is improved. Such a propylene polymer can beobtained by a method of polymerizing components differing in molarweight at multiple stages, a method of using a specific catalyst, amethod of applying a post-processing operation such as crosslinking to apropylene polymer, or the like. Among these methods, the method ofpolymerizing components differing in molar weight at multiple stages ispreferable from a viewpoint of productivity.

[0105] The foregoing resins may contain any generally-used additive,including a filler such as talc, a pigment, an antistatic agent, anantioxidant, and the like. No specific limitation is on the foamingagent applied to the present invention, and any foaming agent, aphysical foaming agent or a chemical foaming agent, can be used.

[0106] [Second Embodiment]

[0107] The following description will explain another embodiment of thepresent invention while referring to FIGS. 7 and 8. The members havingthe same structure (function) as those in the above-mentioned embodimentwill be designated by the same reference numerals and their descriptionwill be omitted.

[0108] A foamed thermoplastic resin sheet manufacturing device of thepresent embodiment is arranged so that an upper wall and a lower wallforming the vacuum chamber 4 of the molding section 9 are composed of afixed upper wall 41 and a fixed lower wall 42 in a front part and themovable upper wall 12 and the movable lower wall 13 in a rear part. Aspace between a wall surface 41 a of the fixed upper wall 41 and a wallsurface 42 a of the fixed lower wall 42 which face each otherconstitutes the expanding zone 7. The wall surfaces 41 a and 42 agradually curve or slant so that a gap therebetween is widened from aninlet to an outlet of the expanding zone 7. A gap between the wallsurfaces 41 a and 42 a is set to W₁ at the inlet, while it is set to W₃at the outlet. The gap W₃ is set, for example, equal to a thickness of afoamed thermoplastic resin sheet 6 which is thinnest amongfrequently-produced foamed thermoplastic resin sheets 6 differing inthickness. Therefore, in this case, when the foamed thermoplastic resinsheet 6 which is thinnest is produced, the gap W₃ becomes equal to thegap W₂. Note that, as described above, the gap W₂ is equivalent to adesired thickness of the foamed thermoplastic resin sheet 6 to beformed.

[0109] Placed under a reduced pressure in the expanding zone 7, thesheet-like foamy thermoplastic resin material 6 a extruded from the die3 expands gradually in the thickness direction to a shape determined bythe wall surfaces 41 a and 42 a. Then, it has a thickness equal to thegap W₃ at the end of the expanding zone 7, and has a thickness equal tothe gap W₂ at the inlet of the cooling zone 8.

[0110] A space between the wall surfaces 12 a and 13 a of the movableupper and lower walls 12 and 13 which face each other constitutes thecooling zone 8. The wall surfaces 12 a and 13 a are plane and aremovably provided so as to approach/separate to/from each other, drivenby the movable wall driving device 23, as described above.

[0111] The movement of the movable upper and lower walls 12 and 13, themovement of the sealing members 31, the pressure reducing operation, andthe like during manufacture of the foamed thermoplastic resin sheet 6are identical to those in the first embodiment, and the state shown inFIG. 7 corresponds to the state shown in FIG. 1(a), while the statesshown in FIGS. 8(a) and 8(b) correspond to the states shown in FIGS.5(a) and 5(b), respectively.

[0112] With the present manufacturing device in which the gap betweenthe wall surfaces 41 a and 42 a is gradually widened, the sheet-likefoamy thermoplastic res in material 6 a extruded from the die 3 issmoothly guided, and hauled by the haul-off machine 5 in a goodcondition. Furthermore, effects achieved by that the wall surfaces 12 aand 13 a are movable and that the sealing members 31 are provided areidentical to those described above.

[0113] [Third Embodiment]

[0114] The following description will explain still another embodimentof the present invention while referring to FIGS. 9 and 10. The membershaving the same structure (function) as those in the above-mentionedembodiments will be designated by the same reference numerals and theirdescription will be omitted.

[0115] The manufacturing device shown in FIG. 9 is equipped with atemperature adjusting zone 51 in the molding section 9, before thevacuum chamber 4. A fixed upper wall 52 and a fixed lower wall 53, aspace between which constitutes the temperature adjusting zone 51, haveinner walls 57, respectively, which are made of a metal with a highthermal conductivity. Inside or outside the inner walls 57, there areprovided temperature adjusting medium paths 54 through which atemperature adjusting medium flows.

[0116] The temperature adjusting zone 51 is a zone for adjusting asurface temperature of the sheet-like foamy thermoplastic resin material6 a extruded from the die 3 to a temperature in a desired temperaturerange. To be more specific, the sheet-like foamy thermoplastic resinmaterial 6 a is heated to a predetermined temperature by heat suppliedfrom the temperature adjusting medium paths 54. The provision of thetemperature adjusting zone 51 enables adjustment of a temperature in theexpanding zone 7 during the expansion, thereby ensuing more stableproduction of the foamed thermoplastic resin sheet 6.

[0117] The set temperature is determined depending on a thermoplasticresin and a foaming agent used, and in the case where the thermoplasticresin is a crystalline resin, the temperature is set not lower than thecrystallization point, and not higher than a temperature of thesheet-like foamy thermoplastic resin material 6 a at the outlet of thedie 3.

[0118] For example, in the case where a polypropylene resin is used as athermoplastic resin, the temperature of the temperature adjustment zone51 is set to 130° C. to 180° C., and the surface temperature of thesheet-like foamy thermoplastic resin material 6 a is controlled so as tofall in a range of ±2° C. to the foregoing set temperature.

[0119] Particularly in the case where the die 3 is a circular die and asheet-like foamy thermoplastic resin material which is obtained bycutting in a lengthwise direction a cylindrical-form foamy thermoplasticresin material extruded from the die 3 is expanded under a reducedpressure, the manufacturing device of a foamed thermoplastic resin sheetpreferably has the temperature adjusting zone 51.

[0120] Incidentally, the means for heating the inner walls 57 is notlimited to the temperature adjusting medium paths 54, and there is nospecific limitation on the means, provided that it is capable ofadjusting temperature and keeping the temperature of the inner walls 57to the set temperature.

[0121] Furthermore, since the temperature of the surfaces of thesheet-like foamy thermoplastic resin material Ga can be more easilycontrolled in the case where the whole surfaces of the sheet-like foamythermoplastic resin material 6 a are in contact with the inner walls 57,it is preferable that the gap between the wall surfaces 52 a and 53 a ofthe inner walls 57 in the temperature adjusting zone 51 is substantiallyequal to W₁, i.e., substantially equal to a thickness of the sheet-likefoamy thermoplastic resin material 6 a extruded from the die 3, and ifpossible, it is more preferable that the gap between the wall surface 52a and 53 a is adjustable. It is, however, not indispensable to make thewhole surfaces of the sheet-like foamy thermoplastic resin material 6 abe in contact with the inner surfaces of the temperature adjusting zone51.

[0122] Furthermore, the manufacturing device may be arranged, as shownin FIGS. 10(a) and 10(b), so that the molding section 9 including thetemperature adjusting zone 51 may be distanced from the die 3. Here, atleast a pair of rolls 55 is preferably provided at an inlet of thetemperature adjusting zone 51 so that the sheet-like foamy thermoplasticresin material 6 a extruded from the die 3 is taken into the temperatureadjusting zone 51 through between the rolls 55. In this manufacturingdevice, a pair of cutters 56 may be provided at the inlet of thetemperature adjusting zone 51, on both sides thereof in the sheet widthdirection as shown in FIG. 10(b), so as to adjust the width of thesheet-like foamy thermoplastic resin material 6 a.

[0123] With the arrangement in which the rolls 55 are provided, it ispossible to adjust the thickness of the sheet-like foamy thermoplasticresin material 6 a before the sheet-like foamy thermoplastic resinmaterial 6 a enters the vacuum chamber 4, thereby facilitating thesliding of the sheet-like foamy thermoplastic resin material 6 a intothe temperature adjusting zone 51.

[0124] The rolls 55 are preferably arranged so that a temperaturethereof can be controlled, and the temperature is appropriately setdepending on a thermoplastic resin and a foaming agent used, as well asa thickness of the foamed thermoplastic resin sheet 6 to be formed. Thetemperature is, however, preferably set not higher than the temperatureof the sheet-like foamy thermoplastic resin material 6 a at the outletof the die 3, and not lower than the set temperature of the temperatureadjusting zone 51.

[0125] Incidentally, the cutters 56 are not necessarily provided, butprovision of the same is preferable for the following reasons. Bycutting, with the cutters 56, the sheet-like foamy thermoplastic resinmaterial 6 a so as to have a width in accordance with a transport pathwidth in the temperature adjusting zone 51, sliding of the sheet-likefoamy thermoplastic resin material 6 a into the temperature adjustingzone 51 is facilitated.

[0126] [Fourth Embodiment]

[0127] The following description will explain still another embodimentof the present invention while referring to FIG. 11. The members havingthe same structure (function) as those in the above-mentionedembodiments will be designated by the same reference numerals and theirdescription will be omitted.

[0128] The device for manufacturing the foamed thermoplastic resin sheet6 may be arranged as shown in FIGS. 12(a) and 12(b). This manufacturingdevice is equipped with a circular die 61, in the place of the die 3which forms into a sheet form the foamy thermoplastic resin materialextruded from the extruder 1 through the head part 2. The circular die61 is intended to process the foamy thermoplastic resin materialextruded from the extruder 1 through the head part 2 into a cylindricalfoamy thermoplastic resin material 6 b.

[0129] Behind the circular die 61, there is provided a cutter 62 whichcuts in the extrusion direction the cylindrical foamed thermoplasticresin material 6 b extruded into the atmosphere through the circular die61, thereby forming it into a developed sheet-like foamy thermoplasticresin material 6 a. Therefore, the cylindrical foamy thermoplastic resinmaterial 6 b is cut out by the cutter 62 thereby becoming the sheet-likefoamy thermoplastic resin material 6 a. Then, it is taken into thevacuum chamber 4 by the rolls 55.

[0130] Incidentally, the means for cutting out the cylindrical foamythermoplastic resin material 6 b is not limited to the cutter 56, butany means may be applicable provided that it is capable of the foregoingcutting operation.

[0131] Furthermore, the aforementioned manufacturing device is providedwith the temperature adjusting zone 51 before the vacuum chamber 4. Thefunction of the temperature adjusting zone 51 is described as above.Further, the foregoing roll 55 and the cutter 56 are not indispensable,though they are provided at the entrance of the temperature adjustingzone 51 so that the arrangement becomes desirable for taking thesheet-like foamy thermoplastic resin material 6 a into the vacuumchamber 4.

[0132] In the manufacturing device as described above, the extruder 1,the die 3, and the vacuum chamber 4 are provided in a row in ahorizontal direction, but alternatively, they may be disposed so thatthe extrusion direction of the die 3 is a downward direction and thatthe vacuum chamber 4 is disposed on a product in such an extrusiondirection.

[0133] Here, the following description will explain a result ofobservation of cell forms of foamed thermoplastic resin sheets 6produced by the foregoing manufacturing devices. As a result of theobservation, it was found that each obtained foamed thermoplastic resinsheet had a foaming ratio of 2.5 or more, and cells existing in aninterior part which extended inward from a depth of 20 percent the wholethickness of the foamed thermoplastic resin sheet from both the surfacesof the sheet respectively in the thickness direction of the same, andfrom a depth of 15 percent the width of the foamed thermoplastic resinsheet from both the side surfaces respectively, satisfied the followingexpressions (1) and (2):

0.5≦D/C≦0.9  (1)

0.5≦E/C≦0.9  (2)

[0134] where C represents a mean cell diameter in the thicknessdirection of the foamed thermoplastic resin sheet, D represents a meancell diameter in the extrusion direction of the same, and E represents amean cell diameter in the width direction of the same.

EXAMPLE 1

[0135] The following description will explain an example of the presentinvention while referring to FIG. 12.

[0136] In the present example, a mixture of polypropylene andpolyethylene was used as the thermoplastic resin, and a mixture ratiothereof, i.e., polypropylene polyethylene, was set to 70 wt %:30 wt %.Further, as a foaming agent and a foaming assistant, 3.5 parts by weightof a 30 wt % masterbatch (base: polyethylene) of a composite foamingagent in which sodium bicarbonate/azodicarbonamide/zinc oxide was9/0.5/0.5 was added.

[0137] The manufacturing device shown in FIG. 1 was used to produce thefoamed thermoplastic resin sheet 6 herein. In the manufacturing device,a single-axis 65 mmφ extruder was used as the extruder 1. Set conditionsof the device are shown in Table 1 below. TABLE 1 DIE TEMPERATURE 175°C.  EXPANDING ZONE TEMPERATURE 60° C. COOLING ZONE TEMPERATURE 30° C.VACUUM CHAMBER PRESSURE 360 mmHg (DIFFERENTIAL PRESSURE) APPARENTFRICTION COEFFICIENT 0.32

[0138] A cross section of the foamed thermoplastic resin sheet 6produced by using the foregoing manufacturing device was observed andcell diameters were measured. It was confirmed that the measurementresult satisfied the aforementioned requirements (the formulas (1) and(2)). The measurement result is shown in Table 2. For a control, a resinwith the same composition and the same foaming agent, extruder 1, anddie 3 as those in the example were used, and a foamed thermoplasticresin sheet was produced by extrusion to an atmospheric pressure. Aresult of observation of a cross section of the obtained foamedthermoplastic resin sheet is shown in the table.

[0139] Incidentally, as measured values of each cell, maximum tangentline distances of the cell in the thickness direction, the extrusiondirection, and the width direction of the foamed thermoplastic resinsheet were used, as shown in FIG. 12.

[0140] Furthermore, (mean cell diameter in the extrusion direction ofthe foamed thermoplastic resin sheet)/(mean cell diameter in thethickness direction of the foamed thermoplastic resin sheet), that is,D/C, was measured by the following method.

[0141] First, an area of 20 cm (in the width direction) x 20 cm (in theextrusion direction) was chosen out of the whole area of the sheetexcluding 15 percent the width of the sheet from both side edges of thesheet, respectively, and at three points in the chosen area, sampleshaving cross sections parallel with the extrusion direction and thethickness direction and samples having cross sections parallel with thewidth direction and the thickness direction were taken out. Then,regarding each sample, a photomicrograph of a cross section parallelwith the extrusion direction in a region whose depth from each surfaceof the sheet exceeds 20 percent the whole thickness of the foamedthermoplastic resin sheet was taken. Regarding more than half of cellsin a 1 mm² square region in each photomicrograph, c (diameter in thethickness direction) and d (diameter in the extrusion direction) weremeasured in the manner shown in FIG. 12. Then, from c₁, c₂, . . . c_(n)and d₁, d₂, . . . d_(n) in all the regions thus obtained, C and D whichare mean values of c and d, respectively, were found, and further, D/Cwas obtained. Here, n satisfies n≧30.

[0142] Furthermore, (mean cell diameter in the width direction of thefoamed thermoplastic resin sheet)/(mean cell diameter in the thicknessdirection of the foamed thermoplastic resin sheet), i.e., E/C wasmeasured by the following method.

[0143] Regarding each of the foregoing three samples, a photomicrographof a cross section parallel with the width direction in a region whosedepth from each surface of the sheet exceeds 20 percent the wholethickness of the foamed thermoplastic resin sheet was taken. Regardingmore than half of cells in a 1 mm² square region in eachphotomicrograph, c (diameter in the thickness direction) and e (diameterin the width direction) were measured in the manner shown in FIG. 12.Then, from c₁, c₂, . . . c_(n) and e₁, e₂, . . . e_(n) in all theregions thus obtained, C and E which are mean values of c and e,respectively, were found, and further E/C was obtained. Here, nsatisfies n≧30.

[0144] Smoothness of surfaces of the foamed thermoplastic resin sheet 6was evaluated by *center line average height Ra indicating surfaceroughness specified by JIS B0601. The center line average height Ra wasmeasured under conditions of a cut-off value of 0.8 mm, a measuredlength of 10 mm, and a driving speed of 0.3 mm/S, and the measuredresult is an average of measured values at 5 points. TABLE 2 MAG- NI-FICA- SUR- TION FACE THICKNESS (DIA- CELL FORM ROUGH- (mm) METER) D/CE/C NESS EXAMPLE 6.8 5.4 0.7 0.8 ∘ CONTROL 2.8 2.7 1.4 1.3 Δ

[0145] It was confirmed that the foamed thermoplastic resin sheet 6 ofthe example having the cell form as described above had a high foamingratio and a great thickness.

[0146] The foamed thermoplastic resin sheet manufacturing device of thepresent invention may be arranged so as to further comprise drivingmeans for moving the movable wall in the directions, wherein (i) the diehas an outlet part which is long in the width direction of the foamythermoplastic resin material, (ii) each of the walls in pair in thefacing-wall section is the movable wall, and (iii) the driving meansmoves the movable walls to positions which are at equal distancesrespectively from a reference position which is a central position ofthe foamy thermoplastic resin material at the outlet part in thethickness direction of the resin material.

[0147] With the foregoing arrangement, driven by the driving means, themovable walls move to positions which are at equal distancesrespectively from a reference position which is a central position, inthe resin material thickness direction, of the foamy thermoplastic resinmaterial at the outlet part of the die.

[0148] Therefore, respective position relations between the outlet partof the die and the walls in pair of the facing-wall section coincidewith each other, thereby ensuring that the formation of the foamedthermoplastic resin sheet between the walls in the vacuum chamber issmoothly carried out. Besides, on both sides to the foamed thermoplasticresin sheet, the reduced pressure in the vacuum chamber is stablymaintained.

[0149] Furthermore, the foamed thermoplastic resin sheet manufacturingdevice of the present invention may be further arranged so as tocomprise temperature adjusting means for adjusting a temperature of thefoamy thermoplastic resin material in the vacuum chamber.

[0150] With the foregoing arrangement, immediately after the expansionof the foamy thermoplastic resin material under a reduced pressure inthe vacuum chamber, a temperature of the foamed thermoplastic resinsheet obtained is adjusted to a desired temperature by the temperatureadjusting means. As a result, the foamed thermoplastic resin sheet canbe kept in the highly foaming state under the reduced pressure.

[0151] Furthermore, the foamed thermoplastic resin sheet manufacturingdevice of the present invention may be further arranged so that (i) thetemperature adjusting means includes a cooling medium path provided ininner walls of the vacuum chamber, and (ii) at least a part of the foamythermoplastic resin material is in contact with the inner walls of thevacuum chamber.

[0152] With the foregoing arrangement, the highly foaming state of thefoamed thermoplastic resin sheet can be more surely maintained.

[0153] Furthermore, the foamed thermoplastic resin sheet manufacturingdevice of the present invention may be further arranged so that thevacuum chamber includes an expanding zone for expanding the foamythermoplastic resin material and a cooling zone for cooling the foamedthermoplastic resin sheet obtained by expansion of the foamythermoplastic resin material under a reduced pressure.

[0154] According the foregoing arrangement, the foamy thermoplasticresin material expands in the expanding zone thereby becoming the foamedthermoplastic resin sheet, and thereafter the sheet is cooled in thecooling zone. As a result, the foamed thermoplastic resin sheet is keptin the highly foaming state under the reduced pressure.

[0155] Furthermore, the foamed thermoplastic resin sheet manufacturingdevice of the present invention may be further arranged so as tocomprise temperature adjusting means for adjusting a temperature of thefoamy thermoplastic resin material in the expanding zone.

[0156] With the foregoing arrangement, immediately after the expansionof the foamy thermoplastic resin material under a reduced pressure inthe expanding zone, a temperature of the foamed thermoplastic resinsheet obtained is adjusted to a desired temperature by the temperatureadjusting means. As a result, the foamed thermoplastic resin sheet canbe kept in the highly foaming state under the reduced pressure.

[0157] Furthermore, the foamed thermoplastic resin sheet manufacturingdevice of the present invention may be arranged so that (i) thetemperature adjusting means includes a cooling medium path provided ininner walls of the vacuum chamber in the expanding zone, and (ii) atleast a part of the foamy thermoplastic resin material is in contactwith the inner walls of the vacuum chamber in the expanding zone.

[0158] With the foregoing arrangement, the highly foaming state of thefoamed thermoplastic resin sheet can be more surely maintained.

[0159] Furthermore, the foamed thermoplastic resin sheet manufacturingdevice of the present invention may be arranged so as to comprise acooling medium path provided in inner walls of the vacuum chamber in thecooling zone, wherein at least a part of the foamed thermoplastic resinsheet is in contact with the inner walls of the vacuum chamber in thecooling zone.

[0160] With the foregoing arrangement, the cooling zone for cooling thefoamed thermoplastic resin sheet can be easily realized, while thefoamed thermoplastic resin sheet can be efficiently cooled.Incidentally, the foamed thermoplastic resin sheet may be partiallybrought into contact with the inner walls of the vacuum chamber, but itis preferable that the whole surfaces of the foamed thermoplastic resinsheet are brought into contact therewith.

[0161] Furthermore, the foamed thermoplastic resin sheet manufacturingdevice of the present invention may be arranged so that the die has anoutlet part which is long in the width direction of the foamythermoplastic resin material and projects to inside of the vacuumchamber.

[0162] Usually, a part of the foamy thermoplastic resin materialextruded from the outlet part of the die tends to get into a crackbetween the movable wall and the die in a state in which the gap betweenthe walls in pair of the facing-wall section which face each other isnarrowed, thereby hindering transport of the foamy thermoplastic resinmaterial. However, this can be avoided by the foregoing arrangementwherein the outlet part of the die projects into the vacuum chamber.

[0163] Thus, as described above, in the foamed thermoplastic resin sheetmanufacturing device of the present invention, the vacuum chamber isarranged so that temperature is more easily adjusted as compared withthe vacuum chamber of the conventional device. Therefore, a foamedthermoplastic resin sheet with its foaming state under a reducedpressure well maintained can be obtained. As a result, a foamedthermoplastic resin sheet which fully enjoys an effect of cell growthdue to pressure reduction thereby having the cells therein grown up inthe thickness direction of the sheet, and has a good appearance.

[0164] Furthermore, the foamed thermoplastic resin sheet manufacturingdevice of the present invention may be arranged so that at least a partof walls enclosing the vacuum chamber is made of a material having aplurality of vacuum ports, and evacuation of the vacuum chamber iscarried out through the material having vacuum ports.

[0165] According to the foregoing arrangement, at least a part of wallsenclosing the vacuum chamber is made of a material having a plurality ofvacuum ports, and evacuation of the vacuum chamber is carried outthrough the material. Therefore, even if, for example, a part of thevacuum ports is clogged up with the foamy thermoplastic resin material,evacuation of the vacuum chamber is ensured. Incidentally, examples ofthe materials having a plurality of vacuum ports include a member onwhich a number of fine slits with a very small width each are formed.

[0166] Furthermore, the foamed thermoplastic resin sheet manufacturingdevice of the present invention may be arranged so that the materialhaving vacuum ports is a porous material.

[0167] With the foregoing arrangement, the aforementioned accident thatthe foamy thermoplastic resin material clogs up a part of the vacuumports thereby hindering the evacuation of the vacuum chamber, forexample, can be more surely avoided.

[0168] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A manufacturing device of a foamed thermoplastic resin sheet, comprising: an extruder for melting and kneading a thermoplastic resin and a foaming agent to form a melted/kneaded mixture, and extruding the melted/kneaded mixture; a die provided at a front end of said extruder, for forming the melted/kneaded mixture into a sheet-like foamy thermoplastic resin material; a vacuum chamber in which the foamy thermoplastic resin material extruded through said die expands under a reduced pressure; and a facing-wall section composed of a pair of walls of said vacuum chamber which face each other in a thickness direction of the foamy thermoplastic resin material, at least one of the walls being a movable wall provided so as to move in directions in which said walls approach and separate each other.
 2. The manufacturing device as set forth in claim 1, further comprising a sealing member provided at least on said movable wall in said facing-wall section, at an outlet of said vacuum chamber, so as to seal a space between said wall and a foamed thermoplastic resin sheet coming out of said vacuum chamber, the foamed thermoplastic resin sheet being obtained as a result of expansion of the foamy thermoplastic resin material under a reduced pressure.
 3. The manufacturing device as set forth in claim 1, further comprising driving means for moving said movable wall in said directions, wherein: said die has an outlet part which is long in the width direction of the foamy thermoplastic resin material; each of said walls in pair in said facing-wall section is said movable wall; and said driving means moves said movable walls to positions which are at equal distances respectively from a reference position, the reference position being a central position of the foamy thermoplastic resin material at said outlet part in the thickness direction of the foamy thermoplastic resin material.
 4. The manufacturing device as set forth in claim 1, further comprising temperature adjusting means for adjusting a temperature of said foamy thermoplastic resin material in said vacuum chamber.
 5. The manufacturing device as set forth in claim 4, wherein: said temperature adjusting means includes a cooling medium path provided in inner walls of said vacuum chamber; and at least a part of the foamy thermoplastic resin material is in contact with the inner walls of said vacuum chamber.
 6. The manufacturing device as set forth in claim 1, wherein said vacuum chamber includes an expanding zone for expanding the foamy thermoplastic resin material, and a cooling zone, provided behind said expanding zone, for cooling the foamed thermoplastic resin sheet obtained by expansion of the foamy thermoplastic resin material under a reduced pressure.
 7. The manufacturing device as set forth in claim 6, further comprising temperature adjusting means for adjusting a temperature of said foamy thermoplastic resin material in said expanding zone.
 8. The manufacturing device as set forth in claim 7, wherein: said temperature adjusting means includes a cooling medium path provided in inner walls of said vacuum chamber in said expanding zone; and at least a part of the foamy thermoplastic resin material is in contact with the inner walls of said vacuum chamber in said expanding zone.
 9. The manufacturing device as set forth in claim 6, further comprising a cooling medium path provided in inner walls of said vacuum chamber in said cooling zone, wherein at least a part of the foamed thermoplastic resin sheet is in contact with the inner walls of said vacuum chamber in said cooling zone.
 10. The manufacturing device as set forth in claim 1, wherein said die has an outlet part which is long in the width direction of the foamy thermoplastic resin material, said outlet part projecting to inside of said vacuum chamber.
 11. The manufacturing device as set forth in claim 1, wherein: at least a part of walls enclosing said vacuum chamber is made of a material having a plurality of vacuum ports; and evacuation of said vacuum chamber is carried out through said material having vacuum ports.
 12. The manufacturing device as set forth in claim 11, wherein said material having vacuum ports is a porous material.
 13. The manufacturing device as set forth in claim 12, wherein said porous material is a porous electrocast shell.
 14. The manufacturing device as set forth in claim 12, wherein said porous material is a sintered alloy.
 15. The manufacturing device as set forth in claim 12, wherein the vacuum port of said porous material has a diameter of not more than 100 μm.
 16. The manufacturing device as set forth in claim 15, wherein said porous material is a porous electrocast shell.
 17. The manufacturing device as set forth in claim 15, wherein said porous material is a sintered alloy.
 18. The manufacturing device as set forth in claim 1, wherein at least a part of surfaces of walls enclosing said vacuum chamber is formed uneven, so as to decrease an area of the sheet in contact with the wall surfaces when the sheet sent from said die to said vacuum chamber is transported through said vacuum chamber.
 19. The manufacturing device as set forth in claim 18, wherein the uneven part of the wall surfaces is plated with polytetrafluoroethylene (Teflon).
 20. The manufacturing device as set forth in claim 18, wherein protuberances in the uneven part have curving surfaces, respectively.
 21. The manufacturing device as set forth in claim 18, wherein: at least a part of the walls enclosing said vacuum chamber is made of a porous material; and evacuation of said vacuum chamber is carried out through said porous material.
 22. A manufacturing device of a foamed thermoplastic resin sheet, comprising: an extruder for melting and kneading a thermoplastic resin and a foaming agent to form a melted/kneaded mixture, and extruding the melted/kneaded mixture; a circular die provided at a front end of said extruder; means for cutting out a cylindrical foamy thermoplastic resin material extruded from said circular die, so as to form the cylindrical foamy thermoplastic resin material into a sheet form; a vacuum chamber in which the sheet-like foamy thermoplastic resin material expands under a reduced pressure; and a facing-wall section composed of a pair of walls of said vacuum chamber which face each other in a thickness direction of the foamy thermoplastic resin material, at least one of the walls being a movable wall provided so as to move in directions in which said walls approach and separate each other. 